The demand for organic and inorganic light emitting devices (LEDs) in lighting and displays is growing. Their overall external quantum efficiency is the product of the internal quantum efficiency and extraction efficiency. The internal quantum efficiency is the number of photons generated per injected electron. The extraction efficiency is the fraction of the generated photons that exits the (O)LED through its front (viewing) face.
Typical OLEDs are made on a transparent substrate, typically glass or plastic, through which the light is emitted. The light generated inside the OLED toward the front (viewing) face of the device is therefore emitted through the organic layers, the transparent indium tin oxide (ITO) anode, and the glass or plastic substrate.
However, there are innate limitations to the light extraction from the OLEDs due to the interface between the organic and indium tin oxide (ITO) anode layers and the glass substrate, and between the glass substrate and air. This results in part of the generated light being reflected back and trapped inside these layers by total internal reflection (TIR). Specifically, the index of refraction of the organic layers (1.7≦norg) and of the ITO (nITO≦2.0) is larger than that of the glass or plastic substrate (nglass˜nplastic˜1.5). Also, the index of refraction of the glass or plastic substrate is higher than that of air (nair=1.03). This results in significant fractions of the emitted light being totally internally reflected back to the organic and ITO layers, and back to the glass substrate, respectively. Almost all of the light trapped in the organic and ITO layers is reabsorbed and consequently lost. Most of the light trapped in the glass or plastic layer is waveguided to the edge of the OLED (glass mode) resulting in edge emissions through the glass. In both cases, the reflected light does not exit through the front of the device. This limits the OLED luminous and power efficiencies.
Indeed, it has been reported that the extraction efficiency, defined as the fraction of the light generated in the device that exits in the front (viewing) direction, in this typical geometry is
                    η        =                                            ∫              0                              2                ⁢                                                                  ⁢                π                                      ⁢                                                  ⁢                                          ⅆ                φ                            ⁢                                                ∫                  0                                      θ                    ⁢                                                                                  ⁢                    c                                                  ⁢                                  sin                  ⁢                                                                          ⁢                  θ                  ⁢                                                                          ⁢                                                            ⅆ                      θ                                        /                                                                  ∫                        0                                                  2                          ⁢                                                                                                          ⁢                          π                                                                    ⁢                                                                                          ⁢                                              ⅆ                        φ                                                                                                                          =                      [                          1              -                              cos                ⁢                                                                  ⁢                θ                ⁢                                                                  ⁢                c                                      ]                                              (        1        )            i.e., only ˜17% for the typical indices given above; ˜53% is trapped in the high-index organic and ITO layers, and ˜30% is waveguided through the glass to the edges of the device as illustrated in FIG. 8. Thus a 30/17=176% enhancement is expected if the light waveguided through the glass is extracted.
There have been many attempts to increase the extraction efficiency by refractive index matching between each layer, using low n materials, and surface texturing or patterning. Among them, surface patterning with a periodic microlens array on the substrate has been developed and studied. The advantage of this method is that it does not change the original performance of the device because the microlens array pattern is fabricated on the blank side of the glass or plastic substrate. Unfortunately, the largest enhancement of the electroluminescence (EL) output compared to conventional ITO-coated glass-based OLEDs with a 7 μm diameter microlens array is only 68%. Construction of these microlens arrays is also difficult and costly.
There is a need, therefore, for a method of enhancing the EL of OLEDs using a microlens array that is economical and commercially viable. Embodiments of the present invention provide such methods and microlens arrays. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.